This measures what is usually the widest point of the fin's outline – the base. The base length impacts the way that a board will turn and drive (accelerate through turns). A longer base provides more surface area to push against the water, so turns are stronger and drive is increased. However, the larger base doesn't permit the board to turn sharply, so if that's what you're after, go for a fin with a shorter base length.http://www.tactics.com/info/guide-to-surfboard-fins